If you don't mind using regular Basic authentication, you can download
mod_auth_apple, which Apple released as open source.
For Digest authentication, OS X Server includes an Apache module to do this (mod_auth_digest_apple), but Apple has declined to release the source to the module.
I've looked through mod_auth_digest and mod_auth_apple, but it's not obvious to a novice Apache coder like me how to mash them together. Perhaps someone smarter could do this?